Sorry once again! If you haven't already gotten your answers I can help you out at some point this morning. It may be better in the future to post in the Double Battle Metagame thread, where there are others along with me who can help you (others, who are not such bad procrastinators).
To your questions: Yes, there is definitely some use in bluffing Beat Up + Justified strategies. I know at least one person did that during VGC2011, for instance. I haven't done it, but I imagine it works best if they are placed together in the top 2 of 6 party slots; obviously, the foe will be aware of it just the same as if Whimsicott and Terrakion were not adjacent, but having them together will work on his psyche, and perhaps give him the impression that he's up against just another generic Beat Up user who doesn't know any better.
Dual-weather isn't very common, and isn't very easy to use, and harder yet to teambuild. I think there's some worth in using it, from the skill with which I've seen it used online, yet it's difficult enough so that I think you should consider using something else first and going for dual-weather only when you're prepared for it, with a specific idea of what you're going to do.
Walling is pretty different in Double Battles. One way of saying it is that it doesn't exist outside of teams based around Minimize Pokemon, like Blissey. What you can do that is more practical is play defensively. Protect, which is available on practically every Pokemon, and Thunder Wave, available on many, are good examples of moves used in tandem. Protect with a slow, vulnerable Pokemon while its partner uses Thunder Wave to change the momentum in battle, for instance. Battles are often an amalgam of switching, Protecting, and status-ing before going in for the kill.
I wrote a three-part warstory about this that kind of illustrates the last sentence, if you want me to link you to it.
There are "degrees" added to Taunt in Gen V, if that's the correct word. It isn't like Protect, that being a very useful move that you just put on your team as a staple and use as a sort of safety-switch. Taunt has dimensions and twists; it can be stopped by Mental Herb, or Protected against, or Fake Out'd, or be predicted and the foe thus uses an attack move instead, or even Taunted before you get off the Taunt. While it IS a very useful move, and I don't want to be caught dead saying it isn't, if you're just starting out, I would build a team that is just generally good with good synergy, and not torture yourself with finding a way to counter every little thing. Insert Taunt into a moveslot when you've found a team that is really, truly a brainchild, but not into a starting team: you'll find you won't know when to use it effectively.
Taunt is very valuable against skilled players, who will likely have some environmental effect or status effect to set up, or commonly a status ailment to inflict. Against the majority of players, you're just going to be tearing them down with stronger, EV-trained Pokemon, so it won't matter, but if you aren't just out to collect wins and want to have great, involved, intelligent fights (and I suspect you do, to be asking me these questions) then yes, Taunt will be an important move.